Zinc primers, both organic and in-organic coatings, are extensively used in the marine and offshore industry and may also be specified for e.g. bridges, containers, refineries, petrochemical industry, power-plants, storage tanks, cranes, windmills and steel structures part of civil structures e.g. airports, stadia, tall buildings. Such coatings may be based on a number of binder systems, such as binder systems based on silicates, epoxy, polyurethanes, cyclized rubbers, phenoxy resin, epoxy ester, urethane alkyd etc.
In zinc primers, zinc is used as a conductive pigment to produce an anodically active coating. Zinc acts as sacrificial anodic material and protects the steel substrate, which becomes the cathode. The resistance to corrosion is dependent on the transfer of galvanic current by the zinc primer but as long as the conductivity in the system is preserved and as long there is sufficient zinc to act as anode the steel will be protected galvanically. Therefore, zinc pigment particles in zinc primers are packed closely together and zinc primers are typically formulated with very high loadings of zinc powder.
Various approaches have been used in order to reduce the zinc loadings in the art. U.S. Pat. No. 4,621,024 discloses coating microspheres with a metal substrate, such as zinc, resulting in an overall reduction in the metal component of the coating. U.S. Pat. No. 6,287,372 discloses further efforts to reduce the amount of zinc dust in the compositions by incorporation of ceramic microspheres. It is further disclosed that the incorporation of ceramic microspheres facilitates thicker coatings without mud cracking.
WO 96/29372 discloses dry coating compositions for dissolving in a solvent in situ, said dry coating compositions containing graphite to avoid hard settling of the coating compositions.
There is, however, still a need for improved corrosion resistance of steel-based metal structures, which is cost-effective and limits the amount of zinc applied to the protective coatings. High dry film thicknesses in zinc silicate coatings are usually a cause of failure of the paint system due to mud cracking. These high thicknesses are particularly difficult to avoid in overlapping areas, such as welding seams, corners, etc. Hence, there is also still a need for zinc silicate protective coatings allowing for thicker layers of coating without mud cracking.
In order to establish sufficient corrosion protection and ensure optimum performance of the coating, it is necessary to specify the requirements for the protection paint system along with the relevant laboratory performance tests to assess its likely durability. The use of new technologies and paint formulations also means coatings being developed with little or no previous track record. This has resulted in more emphasis being placed on accelerated laboratory testing to evaluate coating performance. Many of these accelerated exposure tests will not, within their exposure time show the negative effects visually on intact coated surfaces. Therefore behaviour of the coatings around artificially made damages, e.g. scores, are given significant considerations and many prequalification tests are based amongst others on rust creep and blistering as well as detachment from scores, ISO 12944, NORSOK M-501, ISO 20340, NACE TM 0104, 0204, 0304, 0404, etc. (Weinell, C. E. and S. N. Rasmussen, Advancement in zinc rich epoxy primers for corrosion protection, NACE International, paper no. 07007 (2007)). These accelerated weathering methods seek to intensify the effects from the environment so that the film breakdown occurs more rapidly (Mitchell, M. J., Progress in offshore coatings, NACE International, paper no. 04001 (2004)). The lower the rust creep the better overall anticorrosive performance.